The present invention relates generally to hot cathode ionization pressure gauges. In particular, the invention is a system for controlling the degas operation in hot cathode ionization pressure gauges.
Hot cathode ionization gauges (e.g., Bayard-Alpert gauges) are commonly used to measure very low pressures (e.g., less than 10xe2x88x923 Torr) in vacuum chambers. Gauges of these types are well known and disclosed, for example, in the Harvey U.S. Pat. No. 3,576,465 and the Bills et al. U.S. Pat. No. 5,250,906. Various aspects of these gauges are also described in the following publications: A. Berman, Total Pressure Measurements in Vacuum Technology, pp. 168-171 and 190-193, 1985; J. M. Lafferty, Foundations of Vacuum Science and Technology, pp. 414-419, 1998; A. Roth, Vacuum Technology 2nd ed., pp. 312-319; P. A. Redhead et al., The Physical Basis of Ultrahigh Vacuum, American Institute of Physics, pp. 307-308, 1993; and R. N. Peacock, Total Pressure Gauging Techniques, HPS Division of MKS Instrument, Inc., Boulder, Colo., 1988.
Briefly, hot cathode ionization gauges include a filament, grid and collector that are often enclosed in an envelope. During operation, a voltage (typically about 180 v) is applied to the grid by a grid supply, and a bias voltage (typically about 30 v) is applied to the filament by a filament supply. A electron current set and controlled by the instrument control system (typically about 0.1-10 mA) flows between the filament and grid. Under these operating conditions a current having a magnitude proportional to the pressure in the gauge will flow through the collector. The value of the proportionality constant, known as the gauge constant, is dependant upon a number of factors including the geometry and operating parameters of the gauge and the type of gas in the chamber. The relationship between the pressure being measured and the gauge operating parameters is defined by the following equation.
xe2x80x83P=Ic/KIe
where:
P=pressure being measured
Ic=collector current
Ie=electron current
K=gauge constant
As described in the references listed above, it is periodically necessary to xe2x80x9cdegasxe2x80x9d the gauge in order to enhance its measurement accuracy. The degassing operation removes gasses that have adsorbed onto the grid and other structures of the gauge. Absent the removal of these adsorbed gasses, they can be released into the measuring (ionizing) volume through various electron or ion processes during a measurement. Since this released gas was not initially in the gas phase within the gauge or system volume, its collection would add a spurious component or error term to the measurement.
There are two methods commonly used for performing degas operations. These are known as the electron bombardment (EB) and resistance (I2R) methods. Both methods involve applying increased (over typical measurement mode operating conditions) power levels to the gauge until certain final degas conditions are met (e.g., 3W for a predetermined time period, or a predetermined power to obtain a heat/temperature or color in the grid). This heating is typically sufficient to bring the structure to a visible orange color when viewed in a lighted room. During an electron bombardment method degas operation the grid voltage is increased to a fixed value greater than the usual operating voltage (e.g., by a factor of 2-4 to between 300 v and 600 v) while the electron current Ie is increased to a fixed value greater than the typical operating current (e.g., by a factor of about 10, to about 20 mA). The grid is effectively heated by the impact of electrons during this operation. The power applied to the gauge during a degas operation using the EB method is computed by the following formula:
WEB=Ie(Vgxe2x88x92Vfb)
where:
WEB=power applied to gauge during EB degassing
Ie=electron current
Vg=grid voltage
Vfb=filament bias voltage
During a resistance method degas operation the grid is ohmically heated by passing a current (e.g., about 2A) through the grid. The power applied to the gauge during a degassing operation using the I2R method is computed by the following formula:
W12R=Ig2R
where:
W12R=power applied to gauge during I2R degassing
Ig=grid current
R=resistance of grid
As mentioned above, the adsorbed gas driven off the gauge structures during degas operations adds to the pressure indicated by the gauge. Unfortunately, hot cathode ionization gauges are susceptible to damage if degassed at relatively high pressures (e.g., pressures greater than about 5xc3x9710xe2x88x925 Torr). Damage to a normally operating gauge is therefore possible if the vacuum pumping of the system to which the gauge is mounted is not able to evacuate the gas driven off during the degas operation at a rate sufficient to prevent the pressure from increasing beyond a safe operating threshold. Stripping or removal of coatings on the gauge cathode by glow discharges forming within the tube are an example of the types of damage that can be caused by degassing at relatively high pressures.
To prevent damage of this type, the gauge control system typically includes an automatic xe2x80x9cshut offxe2x80x9d function that continuously monitors the pressure measured by the gauge and either terminates the degas operation or ends the gauge operation altogether if the measured pressure exceeds a predetermined safe upper pressure limit. To help prevent shut-offs under these circumstances, some gauge control systems will gradually increase the degas operation power level (e.g., grid voltage or electron current) to the final degas power value. Even with such a control system function, however, the upper pressure limit can sometimes be exceeded, in which case the system will automatically shut off. Additional pressure measurement inaccuracies and associated complications with the degas operations result from the fact that the gauge constant varies with grid voltage, electron current and gauge operating pressure.
If the degas operation is terminated or the gauge operation ended by a control system of these types, action by an operator is typically required to restart the operation. This need presents substantial disadvantages when the output of the gauge is being monitored and used to control the vacuum system to which it is mounted. If the gauge is turned off, other vacuum system components requiring pressure measurements may be unnecessarily shut off. In situations where the degas operation (but not gauge operation) is terminated, the degas function required for accurate pressure measurements may not have been fully performed. Inaccuracies in subsequent pressure measurements can result. Furthermore, in many cases the operator may not even be aware that there was a degas operation over-pressure shut off since the control system often provides no indication or display to the operator as to whether the degas operation was successfully completed.
There is, therefore, a continuing need for improved degas systems for hot cathode ionization pressure gauges. In particular, there is a need for control systems and methods that enable the gauges to be effectively degassed while minimizing the possibility that the degas operation or the gauge itself will be shut down. To be commercially viable, any such system must be capable of being efficiently implemented. The system will also preferably require little if any operator action after it is started.
The present invention is an efficient-to-implement and effective degas system for hot cathode ionization pressure gauges. One embodiment of the invention includes applying degas power levels to the gauge, determining the gauge pressure and controlling the degas power level as a function of the gauge pressure. Controlling the degas power level can include increasing and decreasing the degas power level as a function of the gauge pressure to prevent the gauge pressure from reaching a predetermined upper pressure limit.
In one preferred embodiment, controlling the degas power level as a function of gauge pressure includes increasing the degas power level if the gauge pressure is less than the upper pressure limit for a predetermined settle time period. The degas power level is decreased if the gauge pressure is greater than the upper pressure limit. The power level increases and decreases can be made in steps.
In another embodiment, controlling the degas power level as a function of the gauge pressure includes monitoring rates of change of increases in the gauge pressure, determining from the rates of change in the gauge pressure whether the gauge pressure will increase to the upper pressure limit, and decreasing the power level increases if it is determined from the rates of change that the gauge pressure will increase to the upper pressure limit.
In yet other embodiments of the invention, the degas operation continues until final degas conditions are met. A display can be actuated to indicate the successful completion of a degas operation. The termination of degas operations if an overpressure situation is detected can also be displayed.